This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present invention. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present invention. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The energy industry has become increasingly interested in capturing deep-water hydrocarbon production opportunities. An approach to potentially enhance the amount of oil recovered from these opportunities is the use of subsea separation systems to treat the streams of hydrocarbons, water, gas, and other materials produced from subsea wells. Subsea separation offers substantial benefits for oil and gas production including (1) reduced flow assurance concerns, (2) reduced pipeline or line sizing, (3) reduced topside facilities, and (4) reduced energy loss resulting from multiphase flow in the lines. Many of these benefits are presently being realized by the oil and gas industry as subsea processing skids are being developed and applied in an increasing number of fields.
While subsea separation is not trivial is shallow waters (<1500 m), it becomes more challenging in deeper water. As water depth increases, the external pressure on a vessel created by the hydrostatic head increases the required wall thickness for the vessels. At depths greater than 1500 m, the vessel wall thickness necessary to withstand the water pressure becomes impractical as the allowable vessel size is limited in diameter by wall thickness and weight. As a result, deep-water subsea separation is a challenge since traditional large diameter separators cannot typically be used.
As understood by those skilled in the art, fluid streams produced from oil and gas wells generally comprise multiphase mixtures of oil, water, gases, sands, and other materials. Typically, the separation of the oil from water requires a large vessel (i.e., gravity separator) that will provide long retention times sufficient to allow the oil and water to separate. However, due to the size and weight constraints noted above, this is not practical for many offshore and subsea applications.
Therefore, it would be beneficial from an economic standpoint for oil production facilities and the associated separation equipment to be reduced in size in terms of weight and footprint. However, the availability of compact oil/water separation devices is limited. In addition to gravity separators, two other types of deep-water separation devices are usually used: electrostatic coalescers and cyclonic separators. As appreciated by those of ordinary skill, coalescence increases the average droplet size of a fluid distributed in a continuous phase. Per Stokes Law, increased droplet size increases the settling speed which in turn allows for faster separation of the liquids in the downstream gravity separator.
There are versions of electrostatic coalescers which are intended to be situated upstream of the gravity separator to enhance coalescence. An electrostatic coalescer generates an electrical field to induce droplet coalescence in water-in-crude-oil multiphase streams. The electric field acts upon water in the stream causing the water droplets to align. Due to their polarized nature, the droplets are attracted and ultimately collide resulting in coalescence. Some compact electrostatic coalescers are designed solely to coalesce and rely on downstream separators to separate the liquid phases.
Like electrostatic coalescers, cyclonic coalescers may also be situated upstream of the gravity separator to enhance coalescence and thus separation. Unlike electrostatic coalescers, cyclonic coalescers mechanically manipulate the flow path of the fluid stream to induce separation. In operation, cyclonic separators swirl the multiphase stream to induce a centrifugal acceleration onto the denser phase droplets. As the denser fluid is pushed to the wall of the cyclonic separators, the droplets of the denser fluid coalesce. Depending on the density difference between the two phases to be separated, conventional cyclonic coalescer designs often require a high value of centrifugal acceleration for the desired coalescence. However, the high centrifugal acceleration causes the dense phase and/or light phase droplets to begin to shatter due to turbulent effects of the stream. For this reason, application of cyclonic coalescers at practical scales outside of the lab environment has been challenged.
The incentives for deep-water subsea separation are well known, as are the challenges. Known techniques fail to meet these challenges. Existing techniques may enable separation of oil/water streams where emulsions are not likely, or where the watercut is low or high, and thus outside of the inversion range of the mixture. However many fields produce oil/water mixtures with emulsion tendencies which stabilize to a higher degree at watercuts near the inversion range. Due to the limited separation time in deepwater subsea separation and existing limitations of compact equipment, it is challenging to achieve separation of oil/water throughout the entire production life of the aforementioned fields without significantly reducing production rates during the inversion range, or accepting a lower quality oil/water separation from the subsea separation system as is normally achieved outside of the inversion range. Thus, there is a need for improvement in this field.